Crosslinked carboxyalkyl cellulose fibers having permanent and non-permanent crosslinks

ABSTRACT

Substantially water-insoluble, water-swellable, non-regenerated, carboxyalkyl cellulose fibers, wherein the fibers have a surface having the appearance of the surface of a cellulose fiber, and wherein the fibers include a plurality of non-permanent intra-fiber metal crosslinks and a plurality of permanent intra-fiber crosslinks; and fiber bundles that include the fibers.

BACKGROUND OF THE INVENTION

Personal care absorbent products, such as infant diapers, adultincontinent pads, and feminine care products, typically contain anabsorbent core that includes superabsorbent in a fibrous matrix.Superabsorbents are water-swellable, generally water-insoluble absorbentmaterials having a liquid absorbent capacity of at least about 10,preferably of about 20, and often up to about 100 times their weight inwater. While the core's liquid retention or storage capacity is due inlarge part to the superabsorbent, the core's fibrous matrix provides theessential functions of liquid wicking, pad strength and integrity, andsome amount of absorbency under load. These desirable properties areattributable to the fact that the matrix includes cellulosic fibers,typically wood pulp fluff in fiber form.

For personal care absorbent products, U.S. southern pine fluff pulp isused almost exclusively and is recognized worldwide as the preferredfiber for absorbent products. The preference is based on the fluffpulp's advantageous high fiber length (about 2.8 mm) and its relativeease of processing from a wetlaid pulp sheet to an airlaid web. However,these fluff pulp fibers can absorb only about 2-3 g/g of liquid (e.g.,water or bodily fluids) within the fibers' cell walls. Most of thefibers' liquid holding capacity resides in the interstices betweenfibers. For this reason, a fibrous matrix readily releases acquiredliquid on application of pressure. The tendency to release acquiredliquid can result in significant skin wetness during use of an absorbentproduct that includes a core formed exclusively from cellulosic fibers.Such products also tend to leak acquired liquid because liquid is noteffectively retained in such a fibrous absorbent core.

The inclusion of absorbent materials in a fibrous matrix and theirincorporation into personal care products is known. The incorporation ofsuperabsorbent materials into these products has had the effect ofreducing the products' overall bulk while at the same time increasingits liquid absorbent capacity and enhancing skin dryness for theproducts' wearers.

A variety of materials have been described for use as absorbentmaterials in personal care products. Included among these materials arenatural-based materials such as agar, pectin, gums, carboxyalkyl starchand carboxyalkyl cellulosic, such as carboxymethyl cellulose.Natural-based materials tend to form gels rather than maintaining asolid form and are therefore not favored in these products. Syntheticmaterials such as polyacrylates, polyacrylamides, and hydrolyzedpolyacrylonitriles have also been used as absorbent materials inpersonal care products. Although natural-based absorbing materials arewell known, these materials have not gained wide usage in personal careproducts because of their relatively inferior absorbent propertiescompared to synthetic absorbent materials such as polyacrylates. Therelatively high cost of these materials has also precluded their use inconsumer absorbent products. Furthermore, many natural-based materialstend to form soft, gelatinous masses when swollen with a liquid. Thepresence of such gelatinous masses in a product's core tends to limitliquid transport and distribution within the core and preventssubsequent liquid insults from being efficiently and effectivelyabsorbed by the product.

In contrast to the natural-based absorbents, synthetic absorbentmaterials are generally capable of absorbing large quantities of liquidwhile maintaining a relatively non-gelatinous form. Synthetic absorbentmaterials, often referred to as superabsorbent polymers (SAP), have beenincorporated into absorbent articles to provide higher absorbency underpressure and higher absorbency per gram of absorbent material.Superabsorbent polymers are generally supplied as particles having adiameter in the range from about 20-800 microns. Due to their highabsorbent capacity under load, absorbent products that includesuperabsorbent polymer particles provide the benefit of skin dryness.Because superabsorbent polymer particles absorb about 30 times theirweight in liquid under load, these particles provide the furthersignificant advantages of thinness and wearer comfort. In addition,superabsorbent polymer particles are about half the cost per gram ofliquid absorbed under load compared to fluff pulp fibers. For thesereasons it is not surprising that there is a growing trend toward highersuperabsorbent particle levels and reduced levels of fluff pulp inconsumer absorbent products. In fact, some infant diapers include 60 to70 percent by weight superabsorbent polymer in their liquid storagecore. From a cost perspective, a storage core made from 100 percentsuperabsorbent particles is desirable. However, as noted above, such acore would fail to function satisfactorily due to the absence of anysignificant liquid wicking and distribution of acquired liquidthroughout the core. Furthermore, such a core would also lack strengthto retain its wet and/or dry structure, shape, and integrity.

Another drawback of synthetic superabsorbent polymers is their lack ofability to biodegrade. The synthetic polymers' non-biodegradability isdisadvantageous with regard to the disposal of used absorbent productscontaining these polymers.

Cellulosic fibers provide absorbent products with critical functionalitythat has, to date, not been duplicated by particulate superabsorbentpolymers. Superabsorbent materials have been introduced in syntheticfiber form seeking to provide a material having the functionality ofboth fiber and superabsorbent polymer particle. However, thesesuperabsorbent fibers are difficult to process compared to fluff pulpfibers and do not blend well with fluff pulp fibers. Furthermore,synthetic superabsorbent fibers are significantly more expensive thansuperabsorbent polymer particles and, as a result, have not competedeffectively for high volume use in personal care absorbent products.

Cellulosic fibers have also been rendered highly absorptive by chemicalmodification to include ionic groups such as carboxylic acid, sulfonicacid, and quaternary ammonium groups that impart water swellability tothe fiber. Although some of these modified cellulosic materials aresoluble in water, some are water-insoluble. However, none of thesehighly absorptive modified cellulosic materials possess the structure ofa pulp fiber, rather, these modified cellulosic materials are typicallygranular or have a regenerated fibril form.

A need exists for a highly absorbent material suitable for use inpersonal care absorbent products, the absorbent material havingabsorptive properties similar to synthetic, highly absorptive materialsand at the same time offering the advantages of liquid wicking anddistribution associated with fluff pulp fibers. Accordingly, there is aneed for a fibrous superabsorbent that combines the advantageous liquidstorage capacity of superabsorbent polymers and the advantageous liquidwicking of fluff pulp fibers. Ideally, the fibrous superabsorbent iseconomically viable for use in personal care absorbent products and isbiodegradable thereby making the disposal of used absorbent productsenvironmentally friendly. The present invention seeks to fulfill theseneeds and provides further related advantages.

SUMMARY OF THE INVENTION

In one aspect, the present invention provides crosslinked, carboxyalkylcellulose fibers are provided. The fibers of the invention aresubstantially water-insoluble, water-swellable, non-regenerated,carboxyalkyl cellulose fibers having a surface having the appearance ofthe surface of a cellulose fiber. The fibers of the invention include aplurality of non-permanent intra-fiber metal crosslinks and a pluralityof permanent intra-fiber crosslinks.

The non-permanent intra-fiber metal crosslinks include multi-valentmetal ion crosslinks. The multi-valent metal ion crosslinks include oneor more metal ions selected from aluminum, boron, bismuth, titanium,zirconium, cerium, and chromium ions, and mixtures thereof.

The permanent intra-fiber crosslinks include covalent crosslinks with anorganic compound having at least two functional groups capable ofreacting with at least one functional group selected from carboxyl,carboxylic acid, and hydroxyl groups. In one embodiment, the organiccompound is a dihalide that provides a diether crosslink. In anotherembodiment, the organic compound is a polycarboxylic acid that providesa diester crosslink.

In one embodiment, the invention provides substantially water-insoluble,water-swellable, non-regenerated, carboxyalkyl cellulose fibers, whereinthe fibers have a surface having the appearance of the surface of acellulose fiber, and wherein the fibers comprise a plurality ofnon-permanent intra-fiber metal crosslinks and a plurality of permanentintra-fiber crosslinks, wherein the permanent intra-fiber crosslinkscomprise covalent crosslinks formed from treatment with1,3-dichloro-2-propanol.

In another aspect of the invention, fiber bundles are provided. Thefiber bundle includes a plurality of substantially water-insoluble,water-swellable, non-regenerated, carboxyalkyl cellulose fibers having asurface having the appearance of the surface of a cellulose fiber. Thefibers include a plurality of non-permanent intra-fiber metal crosslinksand a plurality of permanent intra-fiber crosslinks. The non-permanentintra-fiber metal crosslinks include multi-valent metal ion crosslinks.The multi-valent metal ion crosslinks include one or more metal ionsselected from aluminum, boron, bismuth, titanium, zirconium, cerium, andchromium ions, and mixtures thereof.

DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages of thisinvention will become more readily appreciated as the same become betterunderstood by reference to the following detailed description, whentaken in conjunction with the accompanying drawings, wherein:

FIG. 1A is a scanning electron microscope photograph (1000×) ofcellulose fibers useful for making the representative crosslinkedcarboxymethyl cellulose fibers of the invention;

FIG. 1B is a scanning electron microscope photograph (1000×) ofrepresentative crosslinked carboxymethyl cellulose fibers of theinvention;

FIG. 1C is a scanning electron microscope photograph (1000×) ofregenerated cellulose fibers;

FIG. 2 is a scanning electron microscope photograph (50×) ofrepresentative crosslinked carboxymethyl cellulose fibers of theinvention;

FIG. 3 is a flow chart illustrating a representative method of theinvention for making crosslinked carboxymethyl cellulose fibers andcrosslinked carboxymethyl cellulose fiber bundles; and

FIG. 4 is a device for conducting fluid intake flowback evaluation.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides substantially water-insoluble,water-swellable, crosslinked carboxyalkyl cellulose fibers; andsubstantially water-insoluble, water-swellable, crosslinked carboxyalkylcellulose fiber bundles. Methods for making the substantiallywater-insoluble, water-swellable fibers and fiber bundles are described.

In one aspect, the present invention provides substantiallywater-insoluble, water-swellable, non-regenerated, carboxyalkylcellulose fibers. The fibers have a surface having the appearance of thesurface of a cellulose fiber and include a plurality of non-permanentintra-fiber metal crosslinks and a plurality of permanent intra-fibermetal crosslinks. As can be seen in FIGS. 1B and 2, the fibers of theinvention have irregular surface patterns (including striations, pits,and pores) coextensive with the fibers' surface. The carboxyalkylcellulose fibers of the invention are fibers having superabsorbentproperties. The fibers are water-swellable, water-insoluble fibers thatsubstantially retain a fibrous structure in their expanded,water-swelled state.

The fibers of the invention are cellulosic fibers that have beenmodified by carboxyalkylation and crosslinking. Water swellability isimparted to the fibers through carboxyalkylation and crosslinkingrenders the fibers substantially insoluble in water. The fibers have adegree of carboxyl group substitution effective to provide advantageouswater swellability. The fibers are crosslinked to an extent sufficientto render the fiber water insoluble. The fibers have a liquid absorptioncapacity that is increased compared to unmodified fluff pulp fibers.

The fibers are substantially insoluble in water. As used herein, fibersare considered to be water soluble when they substantially dissolve inexcess water to form a solution, losing their fiber form and becomingessentially evenly dispersed throughout the water solution. Sufficientlycarboxyalkylated cellulosic fibers that are free from a substantialdegree of crosslinking will be water soluble, whereas the fibers of theinvention, carboxyalkylated and crosslinked fibers, are substantiallywater insoluble.

The fibers of the invention are substantially water-insoluble,water-swellable fibers. As used herein, the term “substantiallywater-insoluble, water-swellable” refers to fibers that, when exposed toan excess of an aqueous medium (e.g., bodily fluids such as urine orblood, water, synthetic urine, or 0.9 weight percent solution of sodiumchloride in water), swell to an equilibrium volume, but do not dissolveinto solution.

The water-swellable, water-insoluble fibers of the invention have asurface having the appearance of the surface of a cellulose fiber. Likenative fibers, the fibers have a surface that includes striations, pits,and pores. The fibers of the invention retain the surface structure ofcellulose fibers because the fibers of the invention are prepared bymethods that do not include dissolving the fibers into solution and thenregenerating those fibers from the solution. Fibers that are prepared byregeneration from solution substantially lack typical fiber structurespresent in native fibers. Regenerated fibers lack, among otherstructural features, surface structure (e.g., striations, pits, andpores). FIGS. 1A, 1B, and 1C are photomicrographs comparing the surfacesof representative wood pulp fibers, representative fibers of theinvention (prepared from the wood pulp fibers shown in FIG. 1A), andrepresentative regenerated fibers, respectively. Referring to FIGS. 1Aand 1B, the surfaces of representative wood pulp fibers andrepresentative fibers of the invention are shown to include features(e.g., irregular surface patterns coextensive with the fibers' surface).In contrast, the surface of representative regenerated fiberssubstantially lack such surface structure (see FIG. 1C).

As used herein, the term “regenerated fiber” refers to a fiber that hasbeen prepared by regeneration (i.e., return to solid form) from asolution that includes dissolved fiber. The term “non-regenerated”refers to a fiber that has not been dissolved into solution and thenregenerated (i.e., returned to solid form) from that solution. As notedabove, whereas the non-regenerated fibers of the invention substantiallyretain the surface structure of the cellulose fibers from which they aremade, regenerated fibers do not.

The fibers of the invention include non-permanent intra-fibercrosslinks. The non-permanent intra-fiber crosslink is a metal-cellulosecrosslink formed using a multi-valent metal ion. The non-permanentcrosslinks can unform and reform in use (e.g., dissociate andre-associate on liquid insult in a personal care absorbent product). Thefibers of the invention further include permanent intra-fibercrosslinks. Permanent intra-fiber crosslinks are stable in use and donot dissociate and re-associate on liquid insult in a personal careabsorbent product.

The fibers of the invention are substantially insoluble in water whilebeing capable of absorbing water. The fibers of the invention arerendered water insoluble by virtue of a plurality of intra-fibercrosslinks.

As used herein, the term “non-permanent intra-fiber metal crosslinks”refers to the nature of the crosslinking that occurs within individualfibers of the invention (i.e., intra-fiber) and among and between eachfiber's constituent cellulose polymers.

The fibers of the invention are intra-fiber crosslinked with a metalcrosslink. The metal crosslink arises as a consequence of an associativeinteraction (e.g., bonding) between functional groups on the fiber'scellulose polymers (e.g., carboxy, carboxylate, or hydroxyl groups) anda multi-valent metal species. Suitable multi-valent metal speciesinclude metal ions having a valency of two or greater and that arecapable of forming an associative interaction with a cellulose polymer(e.g., reactive toward associative interaction with the polymer'scarboxy, carboxylate, or hydroxyl groups). The cellulose polymers arecrosslinked when the multi-valent metal species forms an associativeinteraction with functional groups on the cellulose polymer. A crosslinkmay be formed within a cellulose polymer or may be formed between two ormore cellulose polymers within a fiber. The extent of crosslinkingaffects the water solubility of the fibers and the ability of the fiberto swell on contact with an aqueous liquid (i.e., the greater thecrosslinking, the greater the insolubility).

The fibers of the invention include non-permanent intra-fiber metalcrosslinks. As used herein, the term “non-permanent” refers to themetal-cellulose crosslink. Crosslinked cellulose fibers are well knownand it is generally understood that the crosslinks of such fibers aregenerally permanent in nature (i.e., crosslinks that are stable toordinary use conditions, such as cellulose wetting on liquid insultoccurring in a personal care absorbent product). Permanent crosslinksare those that do not dissociate during the fibers' use and aretypically covalent crosslinks derived from reaction of an organiccompound having at least two functional groups capable of reacting withat least one functional group of a cellulose polymer (e.g., a diethercrosslink derived from crosslinking cellulose with a dihalide such as1,3-dichloro-2-propanol, or a diester crosslink derived fromcrosslinking cellulose with citric acid). A non-permanent crosslink is acrosslink that provides a crosslink within or between a fiber'scellulose polymers, but is reactive toward liquid insult. Thenon-permanent crosslinks of the fibers of the present invention can beunformed and reformed on liquid insult. The metal crosslinks of thefibers of the invention have the characteristic of dissociation onliquid insult, which allow the fibers to expand and swell during liquidacquisition. Once liquid acquisition is complete (i.e., insultterminated), re-association between the dissociated multi-valent metalion species and the cellulose polymer occurs to re-establish acrosslink. In such an instance, the new crosslink is formed in fibersnow swollen with acquired liquid. It will be appreciated that theprocess of dissociating and re-associating (breaking and reformingcrosslinks) the multi-valent metal ion and cellulose polymer is dynamicand also occurs during liquid acquisition. By virtue of thenon-permanent crosslinks, the fibers of the invention have the uniqueproperty of maintaining structural integrity while swelling on liquidinsult.

The fibers of the invention include non-permanent intra-fiber metalcrosslinks. The metal crosslinks include multi-valent metal ioncrosslinks that include one or more metal ions selected from aluminum,boron, bismuth, cerium, chromium, titanium, zirconium, and mixturesthereof. In one embodiment, the crosslinks are formed through the use ofan aluminum crosslinking agent. Suitable aluminum crosslinking agentsinclude aluminum acetates, aluminum sulfate, aluminum chloride, andaluminum lactate. Representative aluminum acetates include aluminummonoacetate, aluminum diacetate, aluminum triacetate, aluminumhemiacetate, aluminum subacetate, and mixtures of aluminum acetates madefrom non-stoichiometric amounts of acetate and hydroxide in an organicsolvent that is water miscible. In one embodiment, the aluminumcrosslinking agent is aluminum monoacetate stabilized with boric acid(aluminum acetate, basic, containing boric acid as stabilizer,CH₃CO₂Al(OH)₂.⅓H₃BO₃, Aldrich Chemical Co.). In another embodiment, thealuminum crosslinking agent is prepared immediately prior to use (seeExamples 4 and 5).

The fibers of the invention include non-permanent metal ion intra-fibercrosslinks and permanent intra-fiber crosslinks. Permanent intra-fibercrosslinks are crosslinks that are stable in use (e.g., stable to liquidinsult when in use in a personal care absorbent product, such as aninfant diaper). Permanent intra-fiber crosslinks can be made bycrosslinking the fibers with an organic compound having at least twofunctional groups capable of reacting with at least one functional groupselected from the group consisting of carboxyl, carboxylic acid, andhydroxyl groups. Permanent intra-fiber crosslinks include ether andester crosslinks (e.g., diether crosslinks).

Permanent crosslinks can be incorporated into the fibers of theinvention in several ways: prior to carboxyalkylation; at the same timeas carboxyalkylation; after carboxyalkylation and before treating with amulti-valent metal ion crosslinking agent; or after treating with amulti-valent metal ion crosslinking agent.

In one embodiment, crosslinked carboxyalkyl cellulose fibers of theinvention can be made from crosslinked pulp fibers. The crosslinks ofthe crosslinked cellulose fibers useful in making the carboxyalkylcellulose are crosslinks that are stable (i.e., permanent) to thecarboxyalkylation reaction conditions. A method for making crosslinkedcarboxyalkyl cellulose fibers from crosslinked fibers and subsequentcrosslinking to incorporate non-permanent crosslinks is described inExample 6. Example 6 describes aluminum acetate crosslinked carboxyalkylcellulose made from 1,3-dichloro-2-propanol crosslinked fibers andaluminum acetate crosslinked carboxyalkyl cellulose made from glyceroldiglycidal crosslinked fibers.

In one embodiment, crosslinked carboxyalkyl cellulose fibers of theinvention can be made by treating cellulose fibers with a crosslinkingagent that provides permanent crosslinks and a carboxyalkylating agentduring carboxyalkylation. A method for making crosslinked carboxyalkylcellulose fibers by treating fibers with a crosslinking agent and acarboxyalkylating agent during carboxyalkylation and subsequentcrosslinking to incorporate non-permanent crosslinks is described inExample 7. Example 7 describes treating cellulose fibers with1,3-dichloro-2-propanol and sodium monochloroacetate to providecarboxymethyl cellulose having permanent crosslinks followed bycrosslinking with aluminum chloride to incorporate non-permanentcrosslinks.

Suitable crosslinking agents useful in making ether crosslinks includedihalide crosslinking agents, such as 1,3-dichloro-2-propanol; diepoxidecrosslinking agents, such as vinylcyclohexene dioxide, butadienedioxide, and diglycidyl ethers (e.g., glycerol diglycidal,1,4-butanediol diglycidal, and poly(ethylene glycol diglycidal));sulfone compounds such as divinyl sulfone; bis(2-hydroxyethyl)sulfone,bis(2-chloroethyl)sulfone, and disodium tris(β-sulfatoethyl)sulfoniuminner salt; and diisocyanates.

Other suitable crosslinking agents useful for making permanentcrosslinks include urea-based formaldehyde addition products (e.g.,N-methylol compounds) and polycarboxylic acids.

Suitable urea-based crosslinking agents include methylolated ureas,methylolated cyclic ureas, methylolated lower alkyl substituted cyclicureas, methylolated dihydroxy cyclic ureas, dihydroxy cyclic ureas, andlower alkyl substituted cyclic ureas. Specific preferred urea-basedcrosslinking agents include dimethylol urea (DMU,bis[N-hydroxymethyl]urea), dimethylolethylene urea (DMEU,1,3-dihydroxymethyl-2-imidazolidinone), dimethyloldihydroxyethylene urea(DMDHEU, 1,3-dihydroxymethyl-4,5-dihydroxy-2-imidazolidinone),dimethylolpropylene urea (DMPU), dimethylolhydantoin (DMH),dimethyldihydroxy urea (DMDHU), dihydroxyethylene urea (DHEU,4,5-dihydroxy-2-imidazolidinone), and dimethyldihydroxyethylene urea(DMeDHEU, 4,5-dihydroxy-1,3-dimethyl-2-imidazolidinone).

Polycarboxylic acid crosslinking agents include those described in U.S.Pat. Nos. 5,137,537; 5,183,707; and 5,190,563, describing the use ofC2-C9 polycarboxylic acids that contain at least three carboxyl groups(e.g., citric acid and oxydisuccinic acid) as crosslinking agents.Suitable polycarboxylic acid crosslinking agents include citric acid,tartaric acid, malic acid, succinic acid, glutaric acid, citraconicacid, itaconic acid, tartrate monosuccinic acid, maleic acid,1,2,3-propane tricarboxylic acid, 1,2,3,4-butanetetracarboxylic acid,all-cis-cyclopentane tetracarboxylic acid, tetrahydrofurantetracarboxylic acid, 1,2,4,5-benzenetetracarboxylic acid, andbenzenehexacarboxylic acid. Other polycarboxylic acids crosslinkingagents include polymeric polycarboxylic acids such as poly(acrylicacid), poly(methacrylic acid), poly(maleic acid),poly(methylvinylether-co-maleate) copolymer,poly(methylvinylether-co-itaconate) copolymer, copolymers of acrylicacid, and copolymers of maleic acid. The use of polymeric polycarboxylicacid crosslinking agents such as polyacrylic acid polymers, polymaleicacid polymers, copolymers of acrylic acid, and copolymers of maleic acidis described in U.S. Pat. No. 5,998,511.

Suitable crosslinking agents also include crosslinking agents that arereactive toward carboxylic acid groups. Representative organiccrosslinking agents include diols and polyols, diamines and polyamines,diepoxides and polyepoxides, polyoxazoline functionalized polymers, andaminols having one or more amino groups and one or more hydroxy groups.

In some embodiments, mixtures and/or blends of crosslinking agents canalso be used.

The crosslinking agent can include a catalyst to accelerate the bondingreaction between the crosslinking agent and cellulosic fiber. Suitablecatalysts include acidic salts, such as ammonium chloride, ammoniumsulfate, aluminum chloride, magnesium chloride, and alkali metal saltsof phosphorous-containing acids.

The amount of crosslinking agent applied to the cellulosic fiber willdepend on the particular crosslinking agent and is suitably in the rangeof from about 0.01 to about 10.0 percent by weight based on the totalweight of cellulosic fiber. In one embodiment, the amount ofcrosslinking agent applied to the fibers is in the range from about 1.0to about 8.0 percent by weight based on the total weight of fibers.

In one embodiment, the crosslinking agent can be applied to thecellulosic fibers as an aqueous alcoholic solution. Water is present inthe solution in an amount sufficient swell the fiber to an extent toallow for crosslinking within the fiber's cell wall. However, thesolution does not include enough water to dissolve the fiber. Suitablealcohols include those alcohols in which the crosslinking agent issoluble and the fiber to be crosslinked (i.e., unmodified orcarboxyalkylated cellulosic fiber) is not. Representative alcoholsinclude alcohols that include from 1 to 5 carbon atoms, for example,methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol,s-butanol, and pentanols. In one embodiment, the alcohol is ethanol. Inanother embodiment, the alcohol is methanol.

It will be appreciated that due to their fibers' structure, the fibersof the invention can have a distribution of carboxyl and/or crosslinkinggroups along the fiber's length and through the fiber's cell wall.Generally, there can be greater carboxyalkylation and/or crosslinking onor near the fiber surface than at or near the fiber core. Surfacecrosslinking may be advantageous to improve fiber dryness and provide abetter balance of total absorbent capacity and surface dryness. Fiberswelling and soak time can also effect the carboxyalkylation andcrosslinking gradients. Such gradients may be due to the fiber structureand can be adjusted and optimized through control of carboxyalkylationand/or crosslinking reaction conditions.

The substantially water-insoluble, water-swellable, non-regenerated,carboxyalkyl cellulose fibers are absorbent fibers and may be used in avariety of applications. The fibers of the invention can be incorporatedinto personal care absorbent products (e.g., infant diapers, adultincontinence products, and feminine care products).

Cellulosic fibers are a starting material for preparing the fibers ofthe invention. Although available from other sources, suitablecellulosic fibers are derived primarily from wood pulp. Suitable woodpulp fibers for use with the invention can be obtained from well-knownchemical processes such as the kraft and sulfite processes, with orwithout subsequent bleaching. Pulp fibers can also be processed bythermomechanical, chemithermomechanical methods, or combinationsthereof. A high alpha cellulose pulp is also a suitable wood pulp fiber.The preferred pulp fiber is produced by chemical methods. Ground woodfibers, recycled or secondary wood pulp fibers, and bleached andunbleached wood pulp fibers can be used. Softwoods and hardwoods can beused. Suitable fibers are commercially available from a number ofcompanies, including Weyerhaeuser Company. For example, suitablecellulosic fibers produced from southern pine that are usable with thepresent invention are available from Weyerhaeuser Company under thedesignations CF416, NF405, PL416, FR516, and NB416. Other suitablefibers include northern softwood and eucalyptus fibers. Suitablenon-wood fibers include rye grass fibers and cotton linters.

Cellulosic fibers having a wide range of degree of polymerization aresuitable for forming the fiber of the invention. In one embodiment, thecellulosic fiber has a relatively high degree of polymerization, greaterthan about 1000, and in another embodiment, about 1500 to about 2500.

In one embodiment, the fibers have an average length greater than about1.0 mm. Consequently, the fibers are suitably prepared from fibershaving lengths greater than about 1.0 mm. Fibers having lengths suitablefor preparing the fibers include southern pine, northern softwood, andeucalyptus fibers, the average length of which is about 2.8 mm, about2.0 mm, and about 1.0 mm, respectively.

The fibers of the invention are carboxyalkylated cellulosic fibers. Asused herein, “carboxyalkylated cellulosic fibers” refer to cellulosicfibers that have been carboxyalkylated by reaction of cellulosic fiberswith a carboxyalkylating agent. It will be appreciated that the term“carboxyalkylated cellulosic fibers” include free acid and salt forms ofthe carboxyalkylated fibers. Suitable metal salts include sodium,potassium, and lithium salt, among others. Carboxyalkylated cellulosicfibers can be produced by reacting a hydroxyl group of the cellulosicfiber with a carboxyalkylating agent to provide a carboxyalkylcellulose.

Suitable carboxyalkylating agents include monochloroacetic acid and itssalts, 3-chloropropionic acid and its salts, and acrylamide. Thecarboxyalkyl celluloses useful in preparing the fibers of the inventioninclude carboxymethyl celluloses and carboxyethyl celluloses.

The fibers of the invention can be characterized as having an averagedegree of carboxyl group substitution of from about 0.5 to about 1.5. Inone embodiment, the fibers have an average degree of carboxyl groupsubstitution of from about 0.8 to about 1.2. In another embodiment, thefibers have an average degree of carboxyl group substitution of about1.0. As used herein, the “average degree of carboxyl group substitution”refers to the average number of moles of carboxyl groups per mole ofglucose unit in the fiber. It will be appreciated that the fibers of thepresent invention include a distribution of carboxyl fibers having anaverage degree of carboxyl substitution as noted above.

As noted above, the fibers of the invention are highly absorptive.

The fibers of the invention have a liquid absorbent capacity of fromabout 8 to about 40 g/g as measured by the centrifuge retention capacity(CRC) test described below. In one embodiment, the fibers have acapacity of at least about 20 g/g. In another embodiment, the fibershave a capacity of at least about 25 g/g.

The fibers of the invention have a liquid absorbent capacity of fromabout 30 to about 70 g/g as measured by the free swell capacity testdescribed below. In one embodiment, the fibers have a capacity of atleast about 50 g/g. In another embodiment, the fibers have a capacity ofat least about 60 g/g.

The fibers of the invention have a liquid absorbent capacity of fromabout 10 to about 40 g/g as measured by the absorbency under load (AUL)test described below. In one embodiment, the fibers have a capacity ofat least about 20 g/g. In another embodiment, the fibers have a capacityof at least about 30 g/g.

The fibers of the invention can be formed into pads by, for example,conventional air-laying techniques and the performance characteristicsof those pads determined. An advantageous property of the fibers of theinvention is that pads formed from these fibers demonstrate rapid liquidacquisition times for multiple insults. For certain pads subjected tomultiple insults, liquid acquisition times for subsequent insultsactually decreases. The liquid acquisition times for subsequent insultsfor pads made from fibers of the invention are measured by the fluidintake flowback evaluation (FIFE) described below. The FIFE results forpads formed from the fibers of the invention are presented in Example 3.

In addition to advantageous liquid acquisition, pads formed from thefibers of the invention demonstrate significant strength and integrityafter being subject to multiple insults. Pad wet strength results forpads formed from the fibers of the invention are presented in Example 3.

Methods for making the fibers of the invention are described in Examples3, 6, and 7. The absorbent properties of the fibers are also summarizedin these examples.

In another aspect of the invention, fiber bundles are provided. Thefiber bundles are an aggregate (or plurality) of the fibers of theinvention described above. In the fiber bundles, adjacent fibers are incontact with each other. The bundle is an aggregate of the fibers inwhich contact between adjacent fibers is maintained mechanically by, forexample, friction or entanglement; or chemically by, for example,hydrogen bonding or crosslinking.

The fiber bundle can have a diameter of from about 50 to about 2000 μm,a basis weight of from about 200 to about 2000 g/m², and a density offrom about 0.03 to about 1.5 g/cm³.

Like their component fibers, the fiber bundles of the invention exhibitsignificant absorbent capacity.

The fibers of the invention can be prepared by a method that includescarboxylating and crosslinking cellulose fibers. In one embodiment,cellulosic fibers are carboxyalkylated and then crosslinked. In thismethod, carboxyalkylated cellulosic fibers are treated with an amount ofcrosslinking agent sufficient to render the resulting fiberssubstantially insoluble in water. In another embodiment, cellulosicfibers are crosslinked then carboxyalkylated. In this method,crosslinked cellulosic fibers are carboxyalkylated to render theresulting fibers highly water absorptive. The fibers formed by eithermethod are highly water absorptive, water swellable, and waterinsoluble.

The method includes carboxyalkylating cellulose fibers by treatingcellulose fibers with a carboxyalkylating agent and a crosslinking agentor agents. In the method, the carboxyalkyl cellulose fibers are notdissolved and therefore retain their fibrous form throughout the methodsteps.

In one embodiment, the method further includes drying the substantiallywater-insoluble, water-swellable, carboxyalkyl cellulose fibers.

In one embodiment, the substantially water-insoluble, water-swellable,carboxyalkyl cellulose fibers are fiberized to provide individualizedfibers. In another embodiment, the substantially water-insoluble,water-swellable, carboxyalkyl cellulose fibers are fiberized to providefiber bundles comprising substantially water-insoluble, water-swellable,carboxyalkyl cellulose fibers.

The carboxyalkylating agent can be monochloroacetic acid or its salts,3-chloropropionic acid or its salts, or acrylamide.

The carboxyalkylating medium comprises a mixture of one or more alcoholsand water. In one embodiment, the alcohol is ethanol. In anotherembodiment, the alcohol is isopropanol.

The fibers of the invention include non-permanent intra-fiber crosslinksformed through the use of multi-valent metal ion crosslinking agents.These crosslinking agents include a metal ion selected from aluminum,boron, bismuth, titanium, zirconium, cerium, or chromium ions. Mixturescan also be used. The multi-valent metal ion crosslinking agent isapplied in an amount from about 0.1 to about 10 percent by weight basedon the weight of fibers. The amount of crosslinking agent will depend onthe nature of the crosslinking agent and the desired absorbentproperties in the product fiber.

In one embodiment, the multi-valent metal ion crosslinking agent is analuminum compound. Suitable aluminum crosslinking agents includealuminum acetates, aluminum sulfate, aluminum chloride, and aluminumlactate. Representative aluminum acetates include aluminum monoacetate,aluminum diacetate, aluminum triacetate, aluminum hemiacetate, aluminumsubacetate, and mixtures of aluminum acetates made fromnon-stoichiometric amounts of acetate and hydroxide in an organicsolvent that is water miscible. In one embodiment, the aluminumcrosslinking agent is aluminum monoacetate stabilized with boric acid(aluminum acetate, basic, containing boric acid as stabilizer,CH₃CO₂Al(OH)₂.⅓H₃BO₃, Aldrich Chemical Co.). In another embodiment, thealuminum crosslinking agent is prepared immediately prior to use.

As noted above, in addition to non-permanent metal ion crosslinks, thefibers of the invention also include permanent intra-fiber crosslinks.Permanent intra-fiber crosslinks can be made by crosslinking the fiberswith an organic compound having at least two functional groups capableof reacting with at least one functional group selected from the groupconsisting of carboxyl, carboxylic acid, and hydroxyl groups. Suitablecrosslinking agents for making permanent crosslinks are described above.Representative permanent crosslinks include ether and ester crosslinks.

The permanent crosslinks can be incorporated into the fibers prior to,during, or after carboxyalkylation.

In one embodiment, the method includes treating the cellulose fiberswith a crosslinking agent prior to carboxyalkylating the cellulosefibers. In this method, crosslinked cellulose fibers arecarboxyalkylated. In this embodiment, the carboxyalkylated cellulosefibers made from crosslinked fibers are subsequently treated with amulti-valent metal ion crosslinking agent to impart non-permanentcrosslinks to the fibers.

In one embodiment, the method includes treating the cellulose fiberswith a crosslinking agent at the same time as carboxyalkylating thecellulose fibers. In this method, cellulose fibers are crosslinkedduring carboxylation. In this embodiment, the carboxyalkylated,crosslinked cellulose fibers are subsequently treated with amulti-valent metal ion crosslinking agent to impart non-permanentcrosslinks to the fibers.

In one embodiment, the method includes treating the fibers with acrosslinking agent after carboxyalkylating the cellulose fibers andprior to treating the carboxyalkyl cellulose fibers with a multi-valentmetal ion crosslinking agent.

In another embodiment, the method further includes treating the fiberswith a crosslinking agent after treating the carboxyalkyl cellulosefibers with a multi-valent metal ion crosslinking agent.

The multi-valent metal ion crosslinking agent is applied to the fibersin an amount from about 0.1 to about 10 percent by weight based on theweight of fibers and the crosslinking agent for making permanentcrosslinks (e.g., organic compound) is applied to the fibers in anamount from about 0.1 to about 5 percent by weight based on the weightof fibers. In one embodiment, the multi-valent metal ion crosslinkingagent is applied in an amount from about 1 to about 8 percent by weightbased on the weight of fibers and the crosslinking agent for makingpermanent crosslinks is applied in an amount from about 0.5 to about 2percent by weight based on the weight of fibers.

A schematic diagram illustrating one representative method for makingsubstantially water-insoluble, water-swellable, crosslinked carboxyalkylcellulose fibers and fiber bundles is illustrated in FIG. 3. Thefollowing is a description of one representative method for making thefibers and fiber bundles.

Pulp Preparation

Wood pulp fibers are the starting material for the preparation of thefibers and fiber bundles of the present invention. In a representativemethod, hardwood or softwood chips are cooked in a conventional ormodified continuous digester to provide pulp having a Kappa numberbetween 20 and 40. The kraft pulp can then be delignified in an oxygendelignification reactor and then subsequently partially or fullybleached by conventional bleaching processes (e.g., elementalchlorine-free bleaching) and bleaching sequences (DEopD or DEopDED). Thepulp capillary viscosity produced by the pulping, delignification, andbleaching steps is greater than about 25 cps and the pulp has abrightness of up to about 87% ISO. The bleached pulp at a consistency offrom about 10 to 15% is then dewatered (e.g., press or centrifuge) toprovide pulp at a consistency of 30-35%. The dewatered pulp is thenfurther dried to a consistency of 50-60% (i.e., never-dry dried pulp) or85-90% (air-dried pulp) by, for example, a through-air dryer. The drypulp is then ready for carboxyalkyl cellulose formation.

Carboxyalkyl Cellulose Preparation

High consistency pulp (e.g., 50-90%) is introduced into either a batchor a continuous carboxyalkyl cellulose reactor at about room temperatureunder nitrogen. The pulp fibers are then treated with 50% by weightsodium hydroxide in water (i.e., mercerization) at about 25 degrees for0.5 to 1 hour. The alkalized pulp is then treated with acarboxyalkylation agent in alcohol (e.g., 50% by weight monochloroaceticacid in ethanol) at a temperature of between about 55-75° C. for threeto four hours. During this time the consistency of pulp in the reactoris from about 15 to about 25% with the ratio of alcohol solvent to waterless than about 2. Once the carboxyalkylation (i.e., etherization) iscomplete, the carboxyalkyl cellulose fibers are neutralized by theaddition of acid (e.g., 33% by weight hydrogen chloride in water).

In the process, the carboxyalkyl cellulose (e.g., carboxymethylcellulose, CMC) is produced, having a degree of substitution (DS) offrom about 0.5 to about 1.5. The degree of substitution is defined asthe moles of carboxyl groups introduced to the fiber per mol ofanhydroglucose units. In a continuous process, the alkylization andetherification chemicals are mixed with the pulp in a mixer and themixture is transported to the reactor without stirring. For a batchprocess, the chemicals are mixed with the pulp in the reactor withcontinuous stirring.

As noted above, the carboxyalkyl cellulose preparation includes threestages: (1) alkylization (i.e., mercerization); (2) carboxyalkylation(i.e., etherification); and (3) neutralization and washing.

Representative process conditions for the alkylization stage include atemperature from about 0 to 30° C., a time of about 0.5 to 1.5 hour, aliquor (i.e., alcohol solvent and water) to pulp ratio of from about 2to about 50, a solvent (ethanol or isopropanol) to water ratio of about1 to about 10, and a sodium hydroxide charge rate of about 2-4 mol/molcellulose.

Representative process parameters for the carboxyalkylation reactionstage include a temperature of from about 50 to about 80° C., a processtime of from about 2 to about 4 hours, a liquor to pulp ratio of fromabout 2 to about 20, a solvent to water ratio of from about 1 to about25, and a carboxyalkylating agent (monochloroacetic acid) charge rate ofabout 1 to 2 mol/mol cellulose.

After neutralization, the carboxyalkylated cellulose fibers are washed(e.g., belt washer or centrifuge) with a mixture of an alcohol (e.g.,ethanol) and water (concentration 60-80% mass). In the process, residualsalt is less than 5% mass. During the washing step, acetic acid is usedto neutralize the carboxyalkyl cellulose fibers.

The carboxyalkyl cellulose fibers so produced are ready forcrosslinking.

Crosslinked Carboxyalkyl Cellulose Fiber Preparation

Carboxyalkyl cellulose fibers from the carboxyalkylation reactor areintroduced to a continuous reactor at a consistency of about 30%. In thereactor, the carboxyalkyl cellulose fibers are treated with acrosslinking agent at a consistency of about 5-25% at a temperature offrom about 20 to about 75° C., and for a time of from 0.2 to 2 hours.The temperature and time may depend on the nature of the crosslinkingagent. In a representative crosslinking reactor, the liquor (i.e.,organic solvent and water) to pulp ratio is from about 2 to 20, theorganic solvent to water ratio is from about 1 to about 2, and thecrosslinking agent charge rate is from about 2 to about 7% mass based onthe weight of carboxyalkyl cellulose fibers.

In one embodiment, a crosslinking (permanent crosslinking) reaction iscarried out in the carboxyalkyl cellulose reactor where crosslinking(permanent) occurs substantially simultaneously with carboxyalkylation.Crosslinked carboxyalkyl cellulose fibers (having permanent crosslinks)leaving the crosslinking reactor are then subject to solvent removal(e.g., through the use of steam by a steam stripper) to providesubstantially solvent-free crosslinked carboxyalkyl cellulose fibers.When the crosslinking agent is applied to the carboxyalkyl cellulosefibers in ethanol, the ethanol stripped from the crosslinked fibers canbe returned to an ethanol distillation column for ethanol recovery andrecycling.

Ethanol for solvent in the carboxyalkylation reaction can be fed from anethanol storage tank in liquid communication with an ethanoldistillation column for receiving and recycling ethanol from other stepsin the process.

Ethanol for the crosslinking step as a solvent for the crosslinkingagent can be fed to the crosslinking reactor from ethanol storage.

The substantially ethanol-free fibers can be further defiberized in afluffer (e.g., pin fluffer or shredder) to provide crosslinkedcarboxyalkyl cellulose fibers and related crosslinked carboxyalkylatedcellulose fiber bundles.

Further Crosslinking of Crosslinked Carboxyalkyl Cellulose Fibers

The substantially ethanol-free crosslinked carboxyalkylated cellulosefibers may be optionally further crosslinked by applying a secondcrosslinking agent to the crosslinked carboxyalkylated cellulose fibersand then drying the treated crosslinked carboxyalkylated cellulosefibers to provide crosslinked carboxyalkylated cellulose fibers. Theoptional additional crosslinking occurs during drying, which can becarried out using, for example, fluidized bed dryer, flash dryer, beltconveyor dryer, or drum dryer.

Screening and Packaging Crosslinked Carboxyalkyl Cellulose Fibers

The dried crosslinked carboxyalkyl cellulose fibers and/or fiber bundlescan be screened to select particular size distributions. The final fiberand/or fiber bundle product can be sheeted by air-laying processes andthe final product packaged in rolls. Alternatively, the fiber and/orfiber bundle products can be baled.

Solvent Recovery Salt Recovery and Waste Treatment

The filtrate from the carboxyalkyl cellulose reactor wash and the offgases from the stripper and dryer can be sent to a solvent recoveryprocess. Solvent (e.g., ethanol) can be recovered from the filtrateusing a distillation device. Solvent recovered can be recycled to theprocess. The distillation device residue can be sent to salt recoveryprocess. Residual filtrate can be sent to waste treatment.

The absorbent properties of the crosslinked carboxyalkyl cellulosefibers and fiber bundles can be determined directly or by forming thefibers and/or bundles into pads by air-laying techniques and thentesting the pad performance.

Test Methods Free Swell and Centrifuge Retention Capacities

The materials, procedure, and calculations to determine free swellcapacity (g/g) and centrifuge retention capacity (CRC) (g/g) were asfollows.

Test Materials:

Japanese pre-made empty tea bags (available from Drugstore.com, INPURSUIT OF TEA polyester tea bags 93 mm×70 mm with fold-over flap)(http:www.mesh.nejp/tokiwa/).

Balance (4 decimal place accuracy, 0.0001 g for air-dried superabsorbentpolymer (ADS SAP) and tea bag weights); timer; 1% saline; drip rack withclips (NLM 211); and lab centrifuge (NLM 211, Spin-X spin extractor,model 776S, 3,300 RPM, 120 v).

Test Procedure:

1. Determine solids content of ADS.

2. Pre-weigh tea bags to nearest 0.0001 g and record.

3. Accurately weigh 0.2025 g±0.0025 g of test material (SAP), record andplace into pre-weighed tea bag (air-dried (AD) bag weight). (ADSweight+AD bag weight=total dry weight).

4. Fold tea bag edge over closing bag.

5. Fill a container (at least 3 inches deep) with at least 2 inches with1% saline.

6. Hold tea bag (with test sample) flat and shake to distribute testmaterial evenly through bag.

7. Lay tea bag onto surface of saline and start timer.

8. Soak bags for specified time (e.g., 30 minutes).

9. Remove tea bags carefully, being careful not to spill any contentsfrom bags, hang from a clip on drip rack for 3 minutes.

10. Carefully remove each bag, weigh, and record (drip weight).

11. Place tea bags onto centrifuge walls, being careful not to let themtouch and careful to balance evenly around wall.

12. Lock down lid and start timer. Spin for 75 seconds.

13. Unlock lid and remove bags. Weigh each bag and record weight(centrifuge weight).

Calculations:

The tea bag material has an absorbency determined as follows:

Free Swell Capacity, factor=5.78

Centrifuge Capacity, factor=0.50

Z=Oven dry SAP wt (g)/Air dry SAP wt (g)

Free Capacity (g/g):

$\frac{\begin{matrix}{\left\lbrack {\left( {{{drip}\mspace{14mu} {{wt}(g)}} - {{dry}\mspace{14mu} {bag}\mspace{14mu} {{wt}(g)}}} \right) - \left( {{AD}\mspace{14mu} {SAP}\mspace{14mu} {{wt}(g)}} \right)} \right\rbrack -} \\\left( {{dry}\mspace{14mu} {bag}\mspace{14mu} {{wt}(g)}*5.78} \right)\end{matrix}}{\left( {{AD}\mspace{14mu} {SAP}\mspace{14mu} {{wt}(g)}*Z} \right)}$

Centrifuge Retention Capacity (g/g):

$\frac{\begin{matrix}\left\lbrack {\left( {{{centrifuge}\mspace{14mu} {{wt}(g)}} - {{dry}\mspace{14mu} {bag}\mspace{14mu} {{wt}(g)}} - \left( {{AD}\mspace{14mu} {SAP}\mspace{14mu} {{wt}(g)}} \right)} \right\rbrack -} \right. \\\left( {{dry}\mspace{14mu} {bag}\mspace{14mu} {{wt}(g)}*0.50} \right)\end{matrix}}{\left( {{AD}\mspace{14mu} {SAP}\mspace{14mu} {wt}*Z} \right)}$

Absorbency Under Load (AUL)

The materials, procedure, and calculations to determine AUL were asfollows.

Test Materials:

Mettler Toledo PB 3002 balance and BALANCE-LINK software or othercompatible balance and software. Software set-up: record weight frombalance every 30 sec (this will be a negative number. Software can placeeach value into EXCEL spreadsheet.

Kontes 90 mm ULTRA-WARE filter set up with fritted glass (coarse) filterplate. clamped to stand; 2 L glass bottle with outlet tube near bottomof bottle; rubber stopper with glass tube through the stopper that fitsthe bottle (air inlet); TYGON tubing; stainless steel rod/plexiglassplunger assembly (71mm diameter); stainless steel weight with hole drillthrough to place over plunger (plunger and weight=867 g); VWR 9.0 cmfilter papers (Qualitative 413 catalog number 28310-048) cut down to 80mm size; double-stick SCOTCH tape; and 0.9% saline.

Test Procedure:

1. Level filter set-up with small level.

2. Adjust filter height or fluid level in bottle so that fritted glassfilter and saline level in bottle are at same height.

3. Make sure that there are no kinks in tubing or air bubbles in tubingor under fritted glass filter plate.

4. Place filter paper into filter and place stainless steel weight ontofilter paper.

5. Wait for 5-10 min while filter paper becomes fully wetted and reachesequilibrium with applied weight.

6. Zero balance.

7. While waiting for filter paper to reach equilibrium prepare plungerwith double stick tape on bottom.

8. Place plunger (with tape) onto separate scale and zero scale.

9. Place plunger into dry test material so that a monolayer of materialis stuck to the bottom by the double stick tape.

10. Weigh the plunger and test material on zeroed scale and recordweight of dry test material (dry material weight 0.15 g±0.05 g).

11. Filter paper should be at equilibrium by now, zero scale.

12. Start balance recording software.

13. Remove weight and place plunger and test material into filterassembly.

14. Place weight onto plunger assembly.

15. Wait for test to complete (30 or 60 min)

16. Stop balance recording software.

Calculations:

A=balance reading (g) * −1 (weight of saline absorbed by test material)

B=dry weight of test material (this can be corrected for moisture bymultiplying the AD weight by solids %).

AUL (g/g)=A/B (g 1% saline/1 g test material)

Saturated Retention Capacity

The saturated retention capacity is a measure of the total absorbentcapacity of an absorbent garment, an absorbent structure, containmentmeans and superabsorbent material, or a superabsorbent material. Thesaturated retention capacity is determined as follows. The material tobe tested, having a moisture content of less than about 7 weightpercent, is then weighed and submerged in an excess quantity of the roomtemperature (about 23° C.) 0.9% saline. The material is allowed toremain submerged for 20 minutes. After 20 minutes the material isremoved from the urine and placed on a TEFLON coated fiberglass screenhaving 0.25 inch openings (commercially available from Taconic PlasticsInc. Petersburg, N.Y.) which, in turn, is placed on a vacuum box andcovered with a flexible rubber dam material. A vacuum of 3.5 kilopascals(0.5 pounds per square inch) is drawn in the vacuum box for a period of5 minutes. The material is weighed. The amount of fluid retained by thematerial being tested is determined by subtracting the dry weight of thematerial from the wet weight of the material (after application of thevacuum) and is reported as the saturated retention capacity in grams offluid retained. For relative comparisons, this value can be divided bythe weight of the material to give the saturated retention capacity ingrams of fluid retained per gram of tested material. If material, suchas superabsorbent material or fiber, is drawn through the fiberglassscreen while on the vacuum box, a screen having smaller openings shouldbe used. Alternatively, a piece of the tea bag material described belowcan be placed between the material and the screen and the final valueadjusted for the fluid retained by the material as described below.

When the material to be tested is superabsorbent material, the test isrun as set forth above with the following exceptions. A bag is preparedfrom heal sealable tea bag material (grade 542, commercially availablefrom the Kimberley-Clark Corporation). A six inch by three inch sampleof the material is folded in half and heat sealed along two edges toform a generally square pouch. 0.2 grams of the superabsorbent materialto be tested (in the form of particles having a size within the range offrom about 300 to about 600 μm, and a moisture content of less thanabout 5 weight percent) is placed in the pouch and the third side isheat sealed. The test is performed as described with the amount of thefluid absorbed by the bag material being subtracted from the amount offluid retained by the bag and superabsorbent material. The amount offluid absorbed by the bag material is determined by performing thesaturated retention capacity test on an empty bag.

Fluid Intake Flowback Evaluation Test

The fluid intake flowback evaluation (FIFE) test determines the amountof time required for an absorbent composite to intake a predeterminedamount of liquid. A suitable apparatus for performing the FIFE test isshown in FIG. 4.

The samples for testing are prepared from fibers to be tested bydistributing by hand approximately 2.5 g fiber into a 3 inch circularmold to form a uniform pad. A plunger is placed on top of the pad andthe pad pressed to a final caliper of approximately 2.5 mm. The 3 inchcircular pads including forming tissue on the top and bottom of the padsample (composite 600).

Composite 600 is centered on FIFE test plate 601. Top 602 is then placedonto plate 601 with composite 600 centered under insult cylinder 603.Top 602 weighs 360 g providing a testing load of 0.11 psi on the samplewhen top 602 is in place for the test. Plate 601 and top 602 withcylinder 603 are made from PLEXIGLAS (approximate dimensions of 7inches×7 inches). Insult cylinder 603 has an inner diameter of one inch,a length sufficient to receive at least 15 g liquid, and provides forcommunication of liquid to composite 601.

Prior to testing, the sample (composite 601) is weighed and its weightrecorded, and the sample's bulk is measured at 0.05 psi and recorded.

In the test procedure, the sample (composite 601) is centered on plate601 and top 602 applied. Once the sample is in place and the apparatusassembled, 15 g of 0.9% saline (first insult) is added to cylinder 603.Time zero is the time that the liquid first contacts the sample. Thefirst insult time is measured as the time required for the first addedliquid to be absorbed by the sample (i.e., liquid level drops belowupper forming tissue of sample). After 15 minutes, a second insult isdelivered by adding 15 g of 0.9% saline (second insult) to the cylinderand the sample. The second insult time is measured as the time requiredfor the second added liquid to be absorbed by the sample. After 30minutes, the third insult (15 g of 0.9% saline) is delivered and thethird insult time measured, and after 45 minutes, the fourth insult (15g of 0.9% saline) is delivered and the fourth insult time measured.

The following examples are provided for the purposes of illustrating,not limiting, the present invention.

EXAMPLES Example 1 The Preparation of Pre-Crosslinked Pulp

In this example, the preparation of crosslinked cellulosic pulp isdescribed. The crosslinked cellulosic pulp can be used to make thecarboxyalkyl cellulose fibers of the invention.

120 grams of never-dried northern kraft spruce (NKS) pulp (oven-dried(OD) weight is 40 grams) is mixed in a plastic bag with sodiumhydroxide, if necessary, water for 10 minutes at 10% consistency. Liquidis then pressed from the pulp and collected. Crosslinking agent wasadded to the liquid and then mixed with pulp in the bag. The bag washeated at 85° C. in a water bath for 70 minutes. After reaction, thereacting mixture was diluted with deionized (DI) water, filtered, andrepeated to obtain >25% consistency pre-crosslinked pulp for used forcarboxymethyl cellulose (CMC) preparation.

Table 1 summarizes suitable crosslinking agents useful in makingcarboxyalkyl cellulose from crosslinked pulp.

TABLE 1 The preparation of crosslinked pulp useful for makingcarboxymethyl cellulose fibers. Water 10% Sample g NaOH g Crosslinkingagent DS Control 280 0 0 0.94 1-1 280 8 8 g 10% DCP 0.94 1-2 270 8 2 g10% glycerol diglycidal 0.94 1-3 270 8 2 g 10% PEGDE 0.91 1-4 270 8 4 g10% 1.4 butanediol diglycidal 0.94 1-5 270 0 8 g 10% GA and 2 g 10% AS0.91 DS: degree of carboxyl group substitution DCP:1,3-dichloro-2-propanol. PEGDE: poly(ethylene glycol diglycidal ether).GA: glyoxal. AS: aluminum sulfate (Al₂(SO₄)₃•18H₂O).

Example 2 Morphology of the Representative Crosslinked CarboxymethylCellulose Fibers

In this example, the morphology (e.g., twists) of representativecrosslinked carboxyalkyl cellulose fibers of the invention is described.

The twists per millimeter were counted for the pulp or fiber samples intheir dry condition and in wet condition in a seventy percentethanol/water solution. The sample fibers were distributed on amicroscope slide and the twist count per millimeter was performed bymeasuring the length of one hundred fibers and counting the number oftwists on those fibers. A separate count of fibers with no twists waskept for computing the percent yield. The image analysis system wascalibrated using a two millimeter American Optical scale mounted inglass on a microscope slide.

Twist nodes per millimeter=total number of twists/sum of the lengths. %Yield=100*(1−(Tn/(Tn+100))) where Tn is the number of fibers withouttwists.

TABLE 2 Representative crosslinked carboxymethyl cellulose fibermorphology. Twist per mm % Yield Sample Dry Wet Dry Wet NKS Pulp 3.002.08 96.15 85.47 2-1 3.81 2.58 72.46 53.76 2-2 5.35 2.66 85.47 60.98 2-34.19 2.65 76.34 59.52 2-4 3.18 2.68 79.37 53.48 2-5 3.01 2.16 68.9760.61 Average 3.91 2.55 76.52 57.67 Pilot crosslinked CMC 2.48 2.7564.10 46.51 fibers Laboratory CMC fibers 5.62 2.35 85.47 59.17

The crosslinked carboxymethyl fibers of the invention had higher twistcounts than the starting pulp at dry or wet state. These fibers also hadhigher twist counts than starting carboxymethyl cellulose fibers whenwet, but lower twist counts than the starting carboxymethyl cellulosefibers. The crosslinked carboxymethyl fibers of the invention maintainedtheir twist when wet, while carboxymethyl cellulose fibers withoutcrosslinking lose their twist counts. The crosslinked carboxymethylfibers of the invention prepared by the pilot run (Pilot crosslinked CMCfibers) have lower dry twist count than starting pulp, the crosslinkedcarboxymethyl fibers of the invention prepared by laboratory methods, orthe starting carboxymethyl cellulose fibers, but higher wet twist countthan the starting pulp, the crosslinked CMC from lab, CMC, and lab CMC.

Example 3 The Preparation of Representative Crosslinked CarboxymethylCellulose Fibers and Pads Including the Fibers

In this example, the preparation of representative crosslinkedcarboxymethyl cellulose fibers of the invention and pads including thefibers are described.

409 grams of never-dried carboxymethyl cellulose fibers from high alphasulfite pulp (the carboxymethyl cellulose fibers were neutralized in70/30 ethanol/water, filtered and washed with 70/30 ethanol/water,filtered, then washed with 100% ethanol and filtered to 409 grams) (ovendried 70 grams) was mixed in a solution containing 515 grams of ethanol,960 grams of water, 53.6 grams AA or aluminum acetate dibasic/boric acid(boric acid as stabilizer, 33 percent by weight), and 4.0 grams ofSunrez 747 (a permanent crosslinker) for one hour. After the reaction,the slurry was filtered to obtain 240 grams of wet sample. The samplewas pin mill fluffed to obtain fiber bundle. Part of the wet fiberbundle was oven dried at about 60° C. for one hour to obtain dry productfiber bundles (Sample 3-4 and 3-6). The same procedure was used for samecarboxymethyl cellulose fibers with only 50% of aluminum acetate/boricacid used (Sample 3-5 and 3-7). The fibers were tested for aluminum(Al), and boron (B), and the pads from the fibers bundles were tested byFIFE. Control pads with commercial SAP and fluff (CF 416 or NB416) weremade for FIFE test for comparison. All wet pads were tested for padintegrity. Pads 3-6 and 3-7 were made with a pad former.

Table 3 summarizes the absorbent properties of representativecrosslinked carboxyalkyl cellulose fibers and pads made from the fibers,and fiber metal content.

TABLE 3 Crosslinked carboxymethyl cellulose fibers and pad properties.FIFE insult Free Swell CRC time (seconds) Wet Pad Al/B Sample AA (g/g)(g/g) T1 T2 T3 T4 Strength (ppm) 3-4 100% 60 17 18 29 25 24 strong10700/1700 3-5 50% 50 20 90 70 67 58 3.4 N  7800/1100 3-6 100% 60 17 1648 59 75 medium 10700/1700 3-7 50% 50 20 180 83 180 200 2.6 N  7800/1100

Example 4 Representative Crosslinked Carboxyalkyl Cellulose Fibers:Aluminum Subacetate

This example describes the treatment of carboxymethyl cellulose fiberswith aluminum subacetate, an aluminum crosslinking agent preparedimmediately prior to use, to provide crosslinked carboxyalkyl cellulosefibers. This example describes a method for crosslinking carboxyalkylcellulose fibers with this aluminum crosslinking agent.

7.9 gram of aluminum sulfate hexadecahydrate was dissolved in 69.3 gramsof water and 7 grams of calcium carbonate was added slowly withstirring. After completion of CO₂ evolution, 16 grams of acetic acid wasadded slowly with stirring until CO₂ release is complete. The mixturewas stirred and set for overnight to form a clear solution over a whiteprecipitate. The top layer solution was collected through filtration toobtain 67 grams of clear liquid with a pH of 4.2. Into the liquid, 86grams of ethanol was added and another 14 grams of water was added. Thefinal solution (MA) has a pH of 5.25. 16.5 gram of solution MA was mixedwith 15 grams of ethanol/water (6/4 wt) solution in a spray bottle andthe solution was sprayed evenly on 27 grams of never dried cotton lintercarboxymethyl cellulose fibers with DS of 0.95 in a plastic bag (ODweight CMC is 10 grams). The carboxymethyl cellulose fibers withsolution MA was mixed by hand for half an hour and then dried in aaluminum tray at 66° C. for one hour. The dried product fibers have 4000ppm of aluminum and no detectable boron.

The solution MA has 1800 ppm of aluminum and no boron and an IR spectrumdifferent from aluminum acetate stabilized with boric acid or aluminumacetate basic.

Example 5 Representative Crosslinked Carboxyalkyl Cellulose Fibers:Aluminum Monoacetate

This example describes the treatment of carboxymethyl cellulose fiberswith aluminum subacetate, an aluminum crosslinking agent preparedimmediately prior to use, to provide crosslinked carboxyalkyl cellulosefibers. This example describes a method for crosslinking carboxyalkylcellulose fibers with this aluminum crosslinking agent.

Solution, Reagent and Admixture Preparations

The aluminum acetate solution used in this process is prepared bymodification of the process described in United States Pharmacopoeia (26p 93) for aluminum subacetate topical solution, described as thediacetate, Al(O₂CCH₃)₂OH. In contrast, the solution described herein isfor a solution described as the monoacetate, Al(O₂CCH₃)(OH)₂.

Aluminum acetate solution is prepared as follows:

Aluminum sulfate octadecahydrate (490 g) is dissolved in cold water (560g, 1-10° C.). Calcium carbonate (244 g) is added in portions with mixinguntil a stiff slurry is formed. The slurry is diluted with 113 g coldwater and any remaining CaCO₃ is added. Glacial acetic acid (256 mL) isadded with stirring. The mixture is kept cold for 1-2 hours and thenfiltered under vacuum to give approximately 820 g solution (d=1.0996g/mL at 20° C.). The concentration of aluminum acetate, dibasic in thesolution is 23.4% (w/w). Other solutions of lower concentrations may beproduced from this solution by weight/weight serial dilution. The saltsolution is unstable to heat and must be kept cold. The best results areobtained if the solution is used within 4 hours.

The following is a balanced chemical reaction for the basic chemistryinvolved in making aluminum acetate solution:

Al₂(SO₄)₃+2CH₃CO₂H+3CaCO₃+H₂) →2Al(CH₃CO₂) (OH)₂+3CaSo₄+3CO₂

The chemical reaction above is illustrative only, as the recipe usesmore than three-times the equivalent amount of acetic acid called for bythe stoichiometry given.

Reagents made from aluminum acetate solution are produced as follows:

Reagent 1: Concentrated (23.4% w/w) aluminum acetate, dibasic solution(226 g) is diluted with methanol (620 g) and denatured alcohol (250 g)to afford a cocktail containing 4.8% aluminum acetate, dibasic.

Reagent 2: Diluted (14% w/w) aluminum acetate, dibasic solution (247 g)is diluted with methanol (832 g) and denatured alcohol (325 g) to afforda cocktail containing 2.5% aluminum acetate, dibasic.

Admixtures of the carboxymethyl cellulose fibers and aluminum salts areproduced as follows:

Example 5A

Three samples of carboxymethyl cellulose fibers prepared from NKS pulp(DS about 0.9-1.0) in denatured alcohol (13 g fibers and 53 g alcohol)were treated separately with 260-320 g of Reagent 1 in a container sizedsuch that the fibers were completely immersed in the reagent. Themixtures were covered and allowed to stand with occasional stirring for1 hour. The samples were suction filtered to give a series of sampleswith varying retention ratios (R) of 5, 4 and 3, where R=(total wetweight/(fibers-dry weight). The samples were partially dried in aconvection oven equipped with an induced draft for 10-20 minutes at66-68° C. The samples were then pin-milled and returned to the oven foranother 60-80 minutes.

Example 5B

Three samples of carboxymethyl cellulose fibers in denatured alcohol,each containing 15 g fibers and 62 g alcohol, are treated separatelywith 280-350 g of Reagent 2 in a container sized such that the fiberswere completely immersed in the reagent. The samples are worked up inidentical fashion to those in Example 5A.

Example 6 The Preparation of Representative Crosslinked CarboxymethylCellulose Fibers from Crosslinked Cellulose Fibers

In this example, the preparation of representative crosslinkedcarboxymethyl cellulose fibers of the invention are prepared bycrosslinking carboxymethyl cellulose prepared from crosslinkedcellulose.

The following examples describe the use of crosslinked pulp as astarting material for making carboxyalkyl cellulose (e.g., CMC) that isthen further crosslinked (non-permanent crosslinks) to providesuperabsorbent carboxyalkyl cellulose. The crosslinked pulp useful inmaking superabsorbent carboxyalkyl cellulose is crosslinked with acrosslinking agent that provides crosslinks that are stable to thealkaline conditions of the carboxyalkylation reaction. Suitablecrosslinking agents include those that form ether crosslinks.Representative crosslinking agents that form ether crosslinks include1,3-dichloro-2-propanol (DCP), divinyl sulfone (DVS), glyceroldiglycidal, 1,4-butanediol diglycidal, and poly(ethylene glycoldiglycidal ether) (PEGDE).

Example 6A

The Preparation of Crosslinked Carboxymethyl Cellulose from1,3-Dichloro-2-propanol Crosslinked Cellulose. In this example, thepreparation of crosslinked carboxymethyl cellulose from carboxymethylcellulose prepared from crosslinked pulp (1,3-dichloro-2-propanolcrosslinked pulp) is described. In this method, carboxymethyl celluloseprepared from crosslinked pulp is crosslinked with aluminum acetate.

10 grams of air-dried CMC (DS 0.95) from never-dried crosslinked pulp(1,3-dichloro-2-propanol crosslinked pulp, Sample 1-1 in Example 1) wasimmersed in 100 grams of 75/25 ethanol/water solution with 3% aluminumacetate (dibasic, stabilized with boric acid) for 50 minutes. The slurrywas filtered to a weight of 40 grams. The wet samples were then ovendried at 76° C. for 50 minutes (Sample 13A-1).

The same procedure was followed for a low DS CMC (DS 0.8) from a lowconsistency procedure (Sample 6A-2) and a low DS CMC sample (DS 0.6)from a high consistency procedure (Quantum mixer) (Sample 6A-3) (bothcontrol CMCs are from never-dried PA pulp without pre-crosslinking).

Table 4 summarizes the absorbent properties and metal contents of theproduct crosslinked carboxyalkyl celluloses.

TABLE 4 Representative crosslinked carboxymethyl cellulose fiberproperties. Free swell CRC AUL Al B Sample (g/g) (g/g) (g/g) ppm ppm6A-1 58 29 40 — — 6A-2 46 26 29 — — 6A-3 52 17 32 11350 1570

Example 6B

The Preparation of Crosslinked Carboxymethyl Cellulose from GlycerolDiglycidal Crosslinked Pulp. In this example, the preparation ofcrosslinked carboxymethyl cellulose from carboxymethyl celluloseprepared from crosslinked pulp (glycerol diglycidal crosslinked pulp) isdescribed. In this method, carboxymethyl cellulose prepared fromcrosslinked pulp is crosslinked with aluminum acetate.

15 grams of air-dried CMC (DS 0.95) from never-dried crosslinked pulp(glycerol diglycidal crosslinked pulp, Sample 1-2 in Example 1) wasimmersed in 330 grams of 50/50 ethanol/water solution with 1.5% aluminumacetate (dibasic, stabilized with boric acid) for 50 minutes. The slurrywas filtered to a weight of 60 grams. The wet sample was then oven driedat 76° C. for 50 minutes (Sample 6B-1, pH 6.1). The same procedure wasapplied to CMC with slurry pH adjustment (using NaOH) to provide Sample6B-2 (pH 6.9) and Sample 6B-3 (pH 7.7).

Table 5 summarizes the absorbent properties and metal contents of theproduct crosslinked carboxyalkyl celluloses.

TABLE 5 Representative crosslinked carboxymethyl cellulose fiberproperties Free Swell CRC Sample (g/g) (g/g) 6B-1 54 12 6B-2 54 13 6B-349 22

Example 7 The Preparation of Representative Crosslinked CarboxymethylCellulose Fibers:

Crosslinking with 1,3-Dichloro-2-propanol during Carboxyalkylation andCrosslinking with Aluminum Chloride Post-Carboxyalkylation

This example describes the preparation of representative crosslinkedcarboxymethyl cellulose fibers of the invention that are prepared bytwo-stage crosslinking: (1) permanent crosslink formation using1,3-dichloropropanol during carboxyalkylation and (2) non-permanentcrosslink formation using aluminum chloride post-carboxyalkylation.

This example compares the absorbent properties of two representativecrosslinked carboxyalkyl cellulose fibers of the invention: (1)crosslinked carboxyalkyl cellulose fibers that include non-permanentaluminum crosslinks and (2) crosslinked carboxyalkyl cellulose fibersthat include non-permanent aluminum crosslinks and permanent ethercrosslinks.

The example also demonstrates the effect of crosslinking agent amount,pulp degree of polymerization (DP), and carboxyalkyl cellulose degree ofcarboxyl group substitution (DS) on centrifuge retention capacity (CRC).

The first pulp was a lower alpha (86-88%), lower DP (1600-1700 ASTM)kraft fluff pulp designated NB416 manufactured by Weyerhaeuser Company(Pulp A in Table 7).

The second pulp was a high alpha (95%), high DP (2600 ASTM) sulfitedissolving pulp designated Olympic HV manufactured by WeyerhaeuserCompany (Pulp B in Table 7).

In the method, the pulp was carboxymethylated with or without additionof 1,3,-dichloro-2-propanol (DCP), a crosslinking agent that providespermanent crosslinks. The crosslinking agent (0, 2, or 4 weight % basedon oven-dried pulp) was added together with the monochloro acetic acidduring the carboxymethylation process. Two levels of carboxymethylation(DS) were investigated: (1) 0.65-0.75 and (2) 0.95-1.00.

After the carboxymethylation reaction was complete, the CMC slurry wasneutralized with acetic acid and then washed with ethanol/water mixturesto remove salt. The CMC was washed with 100% ethanol and filtered to aconsistency of about 20%

The washed was then crosslinked (e.g., surface crosslinked with anamount of aluminum chloride (a crosslinking agent that providesnon-permanent crosslinks)) in an ethanol/water slurry. The consistencyof the slurry was about 5% and typically contains 60% ethanol and 40%water. The treated CMC was allowed to soak with the aluminum chloridefor about 1 hour and filtered.

The product crosslinked carboxymethyl cellulose was dried in aforced-air oven at about 65° C. until partially dried and then removedand treated in a pin-fluffer to minimize clumpiness. The crosslinkedcarboxymethyl cellulose was then returned to the oven to complete thedrying.

Once dry, the crosslinked carboxymethyl cellulose may be optionally heattreated at higher temperatures to increase the amount of crosslinking.

Absorbent capacity (CRC) generally decreased with increasing levels ofpermanent crosslinking and aluminum chloride treatment. As permanentcrosslinking levels were increased, less aluminum chloride treatment wasrequired to achieve a given CRC.

With Pulp A, the amount of CRC lost as the permanent crosslinking levelis increased is minimal. A 4% permanent crosslinking level appears bestfor Pulp A. CRC decreases more rapidly with increased permanentcrosslinking for Pulp B; a 2% permanent crosslinking level appears best.

CRC decreases with DS. CRC values are generally below 20 g/g for Pulp Aat 0.75 DS. CRC values for Pulp B are also lower at 0.75 DS than at 0.95DS, but remain above 20 g/g for lower aluminum chloride levels.

At low levels of permanent crosslinking and/or DS, Pulp B (higher DP andalpha pulp) has greater capacity levels than Pulp A (lower DP and alphapulp). At higher levels of permanent crosslinking and high DS, Pulp Atends to have higher capacity.

The composition and absorbent properties (CRC) of representativecrosslinked carboxyalkyl cellulose fibers of the invention aresummarized in Table 7.

The following examples describe the preparation of representativecrosslinked carboxyalkyl cellulose fibers of the invention.

Example 7A

The Addition of a Permanent Crosslinking Agent during the Preparation ofCarboxymethyl Cellulose from Never-Dried Kraft Pulp. This exampledescribes the preparation of carboxymethyl cellulose fibers by permanentcrosslink formation using 1,3-dichloropropanol during carboxyalkylation.

Never-dried kraft pulp (200.0 g, oven dried NB416) was mixed withisopropanol (11.36 L) under nitrogen environment at 0° C. for 30 min. Asodium hydroxide solution (167.25 g in water with a total weight of620.15 g) was added dropwise over 30 minutes and the reaction was leftto stir for 1 h. A solution of monochloroacetic acid (181.50 g) and1,3,-dichloro-2-propanol (8.0 g) in isopropanol (439 ml) was addeddropwise to the stirring pulp over 30 min while the reaction temperaturewas increased to 55° C. The reaction was stirred for 3 h and thenfiltered, the filtered product was placed in 12 L 70/30 methanol/watersolution, and neutralized with acetic acid. The resulting slurry wascollected by filtration, washed one time each with 12 L 70/30, 80/20,and 90/10 ethanol/water solutions and then finally with 100% methanol orethanol to provide the product crosslinked carboxymethyl cellulose(Sample 7A).

Example 7B

The Preparation of Carboxymethyl Cellulose from Never-Dried Kraft Pulp.This example describes the preparation of representative crosslinkedcarboxymethyl cellulose fibers of the invention that are prepared bynon-permanent crosslink formation using aluminum chloridepost-carboxyalkylation.

An aluminum chloride crosslinking solution was prepared by combining143.9 g of 100% denatured ethanol, 131.93 grams of water and 0.408 g ofaluminum chloride hexahydrate. To this solution were added 69.00 g ofethanol wet (21.74% solids) carboxymethylcellulose (prepared asdescribed in Example 1). Based on these proportions, the active aluminumchloride applied to the CMC fiber was 1.5% and the ratio of ethanol towas 60% to 40%. The mixture of CMC fiber and crosslinking agent solutionwas mixed and then allowed to stand at room temperature for 1 hour.After standing the slurry was filtered to a weight 60.59 g. and thenoven dried at 68° C. Mid-way through the drying the sample waspin-fluffed to minimize clumping and then returned to the oven until dryto provide crosslinked carboxymethyl cellulose fiber (Sample 7B).

Example 7C

The Addition of a Permanent Crosslinking Agent during the Preparation ofCarboxymethyl Cellulose from Never-Dried Kraft Pulp. This exampledescribes the preparation of representative crosslinked carboxymethylcellulose fibers of the invention that are prepared by two-stagecrosslinking: (1) permanent crosslink formation using1,3-dichloropropanol during carboxyalkylation and (2) non-permanentcrosslink formation using aluminum chloride post-carboxyalkylation.

An aluminum chloride crosslinking solution was prepared by combining150.08 g of 100% denatured ethanol, 131.93 grams of water and 0.489 g ofaluminum chloride hexahydrate. To this solution were added 62.81 g ofethanol wet (23.88% solids) carboxymethylcellulose (Sample 7A, preparedas described in Example 7A). Based on these proportions, the activealuminum chloride applied to the CMC fiber was 1.8% and the ratio ofethanol to was 60% to 40%. The mixture of CMC fiber and crosslinkingagent solution was mixed and then allowed to stand at room temperaturefor 1 hour. After standing the slurry was filtered to a weight 58.03 g.and then oven dried 68 C. Mid-way through the drying the sample waspin-fluffed to minimize clumping and then returned to the oven until dryto provide a representative crosslinked carboxymethyl cellulose fiber ofthe invention (Sample 7C).

Table 6 summarizes the absorbent properties (CRC) of representativecrosslinked carboxyalkyl cellulose fibers.

TABLE 6 Centrifuge retention capacities for representative crosslinkedcarboxymethyl cellulose fibers. Sample CRC* (g/g) 7B 29.0 7C 21.9

TABLE 7 Representative crosslinked carboxymethyl cellulose compositionand centrifuge retention capacity. AlCl₃ DCP (wgt % wgt (wgt % wgt CRC*Sample CMC) Pulp CMC DS CMC) (g/g) 7-1 1.5% A 0.95 0% 29.0 7-2 2.8% A0.95 0% 18.0 7-3 5.0% A 0.95 0% 12.0 7-4 0.8% A 1.01 2% 31.0 7-5 1.5% A1.01 2% 26.1 7-6 2.5% A 1.01 2% 21.2 7-7 0.5% A 1.00 4% 30.4 7-8 1.0% A1.00 4% 27.3 7-9 1.8% A 1.00 4% 21.9 7-10 1.0% B 0.99 0% 23.0 7-11 2.0%B 0.99 0% 36.5 7-12 4.0% B 0.99 0% 24.1 7-13 0.5% B 0.98 2% 36.6 7-141.3% B 0.98 2% 24.7 7-15 2.0% B 0.98 2% 18.4 7-16 0.4% B 0.99 4% 19.27-17 0.8% B 0.99 4% 19.9 7-18 1.5% B 0.99 4% 16.5 7-19 1.0% A 0.72 0%20.3 7-20 2.0% A 0.72 0% 16.7 7-21 4.0% A 0.72 0% 11.6 7-22 0.5% A 0.682% 17.9 7-23 1.3% A 0.68 2% 16.1 7-24 2.0% A 0.68 2% 14.4 7-25 0.4% A0.71 4% 14.2 7-26 0.8% A 0.71 4% 13.2 7-27 1.5% A 0.71 4% 12.3 7-28 0.8%B 0.68 0% 37.5 7-29 1.8% B 0.68 0% 31.2 7-30 3.8% B 0.68 0% 17.3 7-310.5% B 0.69 2% 22.3 7-32 1.0% B 0.69 2% 20.2 7-33 1.5% B 0.69 2% 18.77-34 0.3% B — 4% 14.6 7-35 0.6% B — 4% 14.0 7-36 1.2% B — 4% 13.0

The Centrifuge Retention Capacity test values set forth in Tables 6 and7 (CRC*) were determined by the following method.

The Centrifuge Retention Capacity (CRC) Test measures the ability of theabsorbent sample to retain liquid therein after being saturated andsubjected to centrifugation under controlled conditions. The resultantretention capacity is stated as grams of liquid retained per gram weightof the sample (g/g). For the fiber samples, the sample to be tested isused as is.

The retention capacity is measured by placing 0.2±0.005 grams of thesample into a water-permeable bag which will contain the sample whileallowing a test solution (0.9 weight percent sodium chloride indistilled water) to be freely absorbed by the sample. A heat-sealabletea bag material, such as that available from Dexter Corporation ofWindsor Locks, Connecticut, U.S.A., as model designation 1234T heatsealable filter paper works well for most applications. The bag isformed by folding a 5-inch by 3-inch sample of the bag material in halfand heat-sealing two of the open edges to form a 2.5-inch by 3-inchrectangular pouch. The heat seals should be about 0.25 inches inside theedge of the material. After the sample is placed in the pouch, theremaining open edge of the pouch is also heat-sealed. Empty bags arealso made to serve as controls. Three samples (e.g., filled and sealedbags) are prepared for the test. The filled bags must be tested withinthree minutes of preparation unless immediately placed in a sealedcontainer, in which case the filled bags must be tested within thirtyminutes of preparation.

The bags are placed between two TEFLON coated fiberglass screens having3 inch openings (Taconic Plastics, Inc., Petersburg, N.Y.) and submergedin a pan of the test solution at 23 degrees Celsius, making sure thatthe screens are held down until the bags are completely wetted. Afterwetting, the samples remain in the solution for about 30±1 minutes, atwhich time they are removed from the solution and temporarily laid on anon-absorbent flat surface. For multiple tests, the pan should beemptied and refilled with fresh test solution after 24 bags have beensaturated in the pan.

The wet bags are then placed into the basket of a suitable centrifugecapable of subjecting the samples to a g-force of about 350. Onesuitable centrifuge is a Heraeus LaboFuge 400 having a water collectionbasket, a digital rpm gauge, and a machined drainage basket adapted tohold and drain the bag samples. Where multiple samples are centrifuged,the samples must be placed in opposing positions within the centrifugeto balance the basket when spinning. The bags (including the wet, emptybags) are centrifuged at about 1,600 rpm (e.g., to achieve a targetg-force of about 350), for 3 minutes. The bags are removed and weighed,with the empty bags (controls) being weighed first, followed by the bagscontaining the samples. The amount of solution retained by the sample,taking into account the solution retained by the bag itself, is thecentrifuge retention capacity (CRC) of the sample, expressed as grams offluid per gram of sample. More particularly, the retention capacity isdetermined as:

${CRC} = \frac{\begin{matrix}{{sample}\text{/}{bag}\mspace{14mu} {wgt}\mspace{14mu} {after}\mspace{14mu} {centrifuge}\mspace{14mu} {empty}\mspace{14mu} {bag}\mspace{14mu} {wgt}} \\{{{{after}\mspace{14mu} {centrifuge}} - {{dry}\mspace{14mu} {sample}\mspace{14mu} {wgt}}}\mspace{14mu}}\end{matrix}}{{dry}\mspace{14mu} {sample}\mspace{14mu} {wgt}}$

The three samples are tested and the results are averaged to determinethe centrifuge retention capacity (CRC). The samples are tested at 23±1degrees Celsius at 50±2 percent relative humidity.

While illustrative embodiments have been illustrated and described, itwill be appreciated that various changes can be made therein withoutdeparting from the spirit and scope of the invention.

1. Substantially water-insoluble, water-swellable, non-regenerated,carboxyalkyl cellulose fibers, wherein the fibers have a surface havingthe appearance of the surface of a cellulose fiber, and wherein thefibers comprise a plurality of non-permanent intra-fiber metalcrosslinks and a plurality of permanent intra-fiber crosslinks.
 2. Thefibers of claim 1, wherein the non-permanent intra-fiber metalcrosslinks comprise multi-valent metal ion crosslinks.
 3. The fibers ofclaim 2, wherein the multi-valent metal ion crosslinks comprise one ormore metal ions selected from the group consisting of aluminum, boron,bismuth, titanium, zirconium, cerium, and chromium ions, and mixturesthereof.
 4. The fibers of claim 1, wherein the non-permanent intra-fibermetal crosslinks comprise aluminum ions.
 5. The fibers of claim 1,wherein the permanent intra-fiber crosslinks are selected from the groupconsisting of ether crosslinks and ester crosslinks.
 6. The fibers ofclaim 1, wherein the permanent intra-fiber crosslinks comprise covalentcrosslinks formed from an organic compound having at least twofunctional groups capable of reacting with at least one functional groupselected from the group consisting of carboxyl, carboxylic acid, andhydroxyl groups.
 7. The fibers of claim 6, wherein the organic compoundis 1,3-dichloro-2-propanol.
 8. Substantially water-insoluble,water-swellable, non-regenerated, carboxyalkyl cellulose fibers, whereinthe fibers have a surface having the appearance of the surface of acellulose fiber, and wherein the fibers comprise a plurality ofnon-permanent intra-fiber metal crosslinks and a plurality of permanentintra-fiber crosslinks, wherein the permanent intra-fiber crosslinkscomprise covalent crosslinks formed from 1,3-dichloro-2-propanol.
 9. Thefibers of claim 8, wherein the non-permanent intra-fiber metalcrosslinks comprise multi-valent metal ion crosslinks.
 10. The fibers ofclaim 9, wherein the multi-valent metal ion crosslinks comprise one ormore metal ions selected from the group consisting of aluminum, boron,bismuth, titanium, zirconium, cerium, and chromium ions, and mixturesthereof.
 11. The fibers of claim 8, wherein the non-permanentintra-fiber metal crosslinks comprise aluminum ions.
 12. A fiber bundle,comprising a plurality of substantially water-insoluble,water-swellable, non-regenerated, carboxyalkyl cellulose fibers, whereinthe fibers have a surface having the appearance of the surface of acellulose fiber, and wherein the fibers comprise a plurality ofnon-permanent intra-fiber metal crosslinks and a plurality of permanentintra-fiber crosslinks.
 13. The fiber bundle of claim 12, wherein thenon-permanent intra-fiber metal crosslinks comprise multi-valent metalion crosslinks.
 14. The fiber bundle of claim 13, wherein themulti-valent metal ion crosslinks comprise one or more metal ionsselected from the group consisting of aluminum, boron, bismuth,titanium, zirconium, cerium, and chromium ions, and mixtures thereof.15. The fiber bundle of claim 12, wherein the non-permanent intra-fibermetal crosslinks comprise aluminum ions.
 16. The fiber bundle of claim12, wherein the permanent intra-fiber crosslinks are selected from thegroup consisting of ether crosslinks and ester crosslinks.
 17. The fiberbundle of claim 12, wherein the permanent intra-fiber crosslinkscomprise covalent crosslinks formed from an organic compound having atleast two functional groups capable of reacting with at least onefunctional group selected from the group consisting of carboxyl,carboxylic acid, and hydroxyl groups.
 18. The fiber bundle of claim 17,wherein the organic compound is 1,3-dichloro-2-propanol.